1. Field of the Invention
The present invention relates to a dielectric ceramic composition having reduction resistance and an electronic device, such as a multilayer ceramic capacitor, using the dielectric ceramic composition.
2. Description of the Related Art
A multilayer ceramic capacitor as an electronic device is required to have a high specific permittivity, long lifetime of insulation resistance IR and preferable DC bias characteristics (change of specific permittivity over time is a little) and, moreover, required to have preferable temperature characteristics. Particularly, depending on the use object, the temperature characteristics are required to be flat under severe conditions. In recent years, multilayer ceramic capacitors have come to be used in a variety of electronic apparatuses mounted in engine compartments of automobiles, such as an electronic control unit (ECU), a crank angle sensor and an Anti Lock Brake System (ABS) module. Since these electronic apparatuses are for stable engine controlling, drive controlling and brake controlling, preferable temperature stability of the circuit is required.
Environments of using the electronic apparatuses are expected to become about −20° C. or lower in winter in cold climates and about +130° C. or higher after the engine starts in summer. Recently, there is a tendency of reducing a wire harness for connecting the electronic apparatus and an apparatus to be controlled thereby and the electronic apparatus may be provided outside of a vehicle, therefore, environments for the electronic apparatuses have been getting more severe. Accordingly, a capacitor used by the electronic apparatuses is required to have flat temperature characteristics in a wide temperature range. Specifically, it is not sufficient when the capacitor-temperature characteristics only satisfy the X7R characteristic of EIA standard (−55 to 125° C. and ΔC/C=within ±15%), and a dielectric ceramic composition satisfying the X8R characteristics of EIA standard (−55 to 150° C. and ΔC/C=within ±15%) is required.
There are some proposals on dielectric ceramic compositions satisfying the X8R characteristics.
In the Japanese Unexamined Patent Publication No. 10-25157 and the Japanese Unexamined Patent Publication No. 9-40465, shifting of a Curie's temperature to the high temperature side by substituting Ba in BaTiO3 by Bi and Pb, etc. has been proposed to satisfy the X8R characteristics in a dielectric ceramic composition including BaTiO3 as the main component. Also, there are proposals of satisfying the X8R characteristics by selecting a composition of BaTiO3+CaZrO3+ZnO+Nb2O5 system (the Japanese Unexamined Patent Publication No. 4-295048, the Japanese Unexamined Patent Publication No. 4-292458, the Japanese Unexamined Patent Publication No. 4-292459, the Japanese Unexamined Patent Publication No. 5-109319 and the Japanese Unexamined Patent Publication No. 6-243721).
However, since all of these composition systems use a volatile Pb, Bi and Zn, firing in the air or other oxidizing atmosphere becomes a premise. Therefore, there is a disadvantage that inexpensive base metals, such as Ni, cannot be used for internal electrodes of the capacitor and expensive precious metals, such as Pd, Au and Ag, have to be used.
On the other hand, the present applicant has already proposed a dielectric ceramic composition described below (the Japanese Unexamined Patent Publication No. 2000-154057) for the purpose of having a high permittivity, satisfying the X8R characteristics, and enabling firing in a reducing atmosphere. The dielectric ceramic composition described in the Japanese Unexamined Patent Publication No. 2000-154057 comprises at least BaTiO3 as the main component; a first subcomponent including at least one kind selected from MgO, CaO, BaO, SrO and Cr2O3; a second subcomponent expressed by (Ba, Ca)xSiO2+x (note that x=0.8 to 1.2); a third subcomponent including at least one kind selected from V2O5, MoO3 and WO3; and a fourth subcomponent including an oxide of R1 (note that R1 is at least one kind selected from Sc, Er, Tm, Yb and Lu); and ratios of the respective subcomponents with respect to 100 moles of the main component are the first subcomponent: 0.1 to 3 moles, second subcomponent: 2 to 10 moles, third subcomponent: 0.01 to 0.5 mole, and fourth subcomponent: 0.5 to 7 moles (note that the number of moles of the fourth subcomponent is a ratio of R1 alone).
Also, the present applicant has already proposed a dielectric ceramic composition described below (the Japanese Unexamined Patent Publication No. 2001-192264). The dielectric ceramic composition described in the Japanese Unexamined Patent Publication No. 2001-192264 includes at least a main component including barium titanate, a first subcomponent including at least one kind selected from MgO, CaO, BaO, SrO and Cr2O3; a second subcomponent including an oxide silicon as a main ingredient; a third subcomponent including at least one kind selected from V2O5, MoO3 and WO3; a fourth subcomponent including an oxide of R1 (note that R1 is at least one kind selected from Sc, Er, Tm, Yb and Lu); and a fifth subcomponent including CaZrO3 or CaO+ZrO2; and ratios of the respective subcomponents with respect to 100 moles of the main component are the first subcomponent: 0.1 to 3 moles, second subcomponent: 2 to 10 moles, third subcomponent: 0.01 to 0.5 mole, fourth subcomponent: 0.5 to 7 moles (note that the number of moles of the fourth subcomponent is a ratio of R1 alone) and fifth subcomponent: 0<fifth subcomponent≦5 moles.
In all of the above patent applications filed by the present applicant, a ratio of the first subcomponent, such as MgO, with respect to 100 moles of the main component is 0.1 mole or higher.
Also, the present applicant has already proposed a dielectric ceramic composition described below (the Japanese Unexamined Patent Publication No. 2002-255639). The dielectric ceramic composition described in the Japanese Unexamined Patent Publication No. 2002-255639 comprises a main component including barium titanate, a first subcomponent including an oxide of AE (note that AE is at least one kind selected from Mg, Ca, Ba and Sr); and a second subcomponent including an oxide of R (note that R is at least one kind selected from Y, Dy, Ho and Er); and ratios of the respective subcomponents with respect to 100 moles of the main component are the first subcomponent: 0<first subcomponent<0.1 mole and second subcomponent: 1 mole<second subcomponent<7 moles.
According to the techniques in the Japanese Unexamined Patent Publication No. 2000-154057, the Japanese Unexamined Patent Publication No. 2001-192264 and the Japanese Unexamined Patent Publication No. 2002-255639 by the present applicant explained above, it is possible to obtain a high permittivity, the X8R characteristics are satisfied, and firing in a reducing atmosphere is possible.
However, although the X8R characteristics are satisfied, the IR temperature dependency from the room temperature to a high temperature range was poor and the practical use as a product has sometimes become difficult.